This invention generally relates to packages for power circuits, and is specifically concerned with a co-fired ceramic package that is heat conductive but electrically insulative.
Packages for hermetically sealing power hybrid circuits are known in the prior art. Such packages are typically used in airborne radar system to protect delicate, heat generating power circuits from thermal overload and mechanical shock. Such packages include a housing formed from a heat conductive metal such as copper or a copper alloy that has been gold-plated. The gold-plated base of the housing receives one or more ceramic substrates. Each substrate includes a network of conductive members on its top surface, and an array of discrete electronic components and chips are soldered on to various junctions of the conductor network to complete the circuit. The bottom of the substrate is in turn mechanically and thermally connected to the base of the housing by indium-lead solders. A plurality of terminal posts provide access to the power circuit disposed on the substrates. These posts pass through the walls of the metallic housing, and terminate in thin lead wires which are in turn soldered onto various junctions within the circuit. To prevent the terminal posts from contacting the metallic housing and short circuiting, each post is mounted by way of a glass post seal. To hermetically seal the circuit from the ambient atmosphere, the housing is filled with an inert gas such as nitrogen, and a metallic cover (which again may be copper or a copper alloy) is sealingly braised or soldered around the upper edge of the housing.
While such prior art hermetic packages are generally capable of protecting delicate, miniaturized hybrid power circuits from mechanical shock and thermal overload, the applicants have noted a number of areas where the performance of these packages could be improved. For example, the applicants have noted that the heat dissipating abilities of such packages is hindered by the ceramic-to-metal interfaces between the bottom surface of each ceramic substrate and the base of the housing to which this bottom surface is soldered. The applicants have also noted that the indium used in the ceramic-to-metal interfacing solders tends to form a gold-indium intermetallic compound with the gold plating on the housing which is very brittle. This brittleness, coupled with the stress generated by differential thermal expansion between the ceramic substrates and the base of the metallic housing could cause breakage of the solder bond that secures the substrate, thereby detaching the substrate from the base both mechanically and thermally, and subjecting the wire leads within the housing to relatively high levels of stress. Further, the applicants have noted that the glass post seals that insulatively mount the terminal posts through the walls of the housing are difficult to manufacture, and are one of the areas of the package most apt to mechanical failure. Such failure can break the hermetic seal of the package, and may even cause short-circuiting to occur between the various terminal posts of the circuit.
Clearly, a hermetic package for miniaturized hybrid power circuits is needed which offers improved performance over prior art packages in the areas of thermal conductivity, and mechanical and electrical reliability. It would further be desirable if such a package were smaller in size, lighter in weight and easier to manufacture than known prior art packages and did not rely upon glass seals to insulatively mount terminals through the walls of the package housing.